Liquid crystal display devices are put in practical use in various application fields with taking advantage of the characteristics thereof such as lightweight, a thin outline, and less power consumption.
Twisted nematic (TN) type liquid crystal display devices now widely utilized in the market have a structure in which liquid crystal material thereof having optically positive refractive index anisotropy is arranged between the substrates in such a manner as to be twisted by approximately 90 degrees, so that optical rotatory property of the incident light is regulated upon controlling the twisted alignment. Although the TN type liquid crystal display device can be manufactured comparatively easily, it is not suitable to display moving pictures such as TV images because the viewing angle is narrow and the response speed is slow.
For improving the viewing angle and the response speed, on the other hand, the OCB type liquid crystal display device has been focused on. The OCB type liquid crystal display device, which has a liquid crystal material capable of forming bend alignment sealed between the substrates, has some advantages that the response speed can be improved by one digit in comparison with the TN type liquid crystal display device and furthermore the viewing angle can be widened because it is optically self-compensated thanks to the alignment condition of the liquid crystal material. When image display is carried out with use of the OCB type liquid crystal display device, it is considered upon controlling birefringence and combining with an alignment layer that the light is shut off (the black display) in the condition where a higher voltage is applied thereto, or passes therethrough (the white display) in the condition where a lower voltage is applied thereto. In this case, it is well known that phase difference of the liquid crystal layer in the black display condition is compensated and the transmittance is sufficiently diminished upon, for example, combining with a single-axial phase difference plate.
Though liquid crystal molecules are arranged along the direction of electric field (in the direction normal to the substrate) by applying a higher voltage in the black display condition, liquid crystal molecules in the vicinity of the substrate are not arranged in the normal direction to the substrate due to the interaction with the orientation film, so that the light is affected by the phase difference in the specified direction. As a result, when observation is carried out from the direction normal to the substrate (the front direction of the display screen), the transmittance in the black display condition cannot be sufficiently decreased, so that the above phenomenon gives rise to deterioration of the contrast. As disclosed, for example, in the Patent Document 1, combining with a hybrid arranged optically negative phase difference plate is known as a measure to compensate sufficiently the black display or gray scale characteristicts for the observation in the front direction and moreover the oblique direction. Furthermore, the Patent Document 2 describes that the thickness of liquid crystal layer of each color region is varied to improve the brightness of each color pixel for the general TN type liquid crystal display device.
Patent Document 1: Japanese Laid-open Patent Publication Hei10-197862
Patent Document 2: Japanese Laid-open Patent Publication 2003-5204